Once again, Tom Clancy manages to add new twists to the alternate U. S. history he initiated in The Hunt for Red October. In The Sum of All Fears, the center of conflict is the perpetual hot spot the Mideast, where a nuclear weapon falls into the hands of terrorists just as peace seems possible. Clancy realistically paints an almost unthinkable scenario-the bomb is planted on American soil in the midst of an escalation in…